In analytical chemistry there are a range of sophisticated techniques available such as chromatography, mass spectrometry and other spectroscopy techniques but rarely can the sample be introduced directly into the instrument. Some modification of the sample is usually required, for example removal of interfering matrix, elimination of components that will interfere in the analysis, concentration of the sample, or switching the matrix or solution in which the sample components of interest are dissolved.
There are a variety of techniques involved in sample preparation but amongst the most common are filtering, targeted pre-separation to simplify the sample, concentration of the sample and changing the matrix. It has been reported that 40% of all analytical sample preparation requires Solid Phase Extraction (SPE) and 60% of all analytical sample preparation requires sample filtering during the procedure.
Syringes are used in many areas of laboratories including sample preparation for instrumental analysis. Fundamentally a syringe fulfils a triple role as a pump for displacing fluids, as a metering device for accurately determining the precise volume dispensed and the rate it is dispensed, and as a transport device. Syringes are used manually by hand or motor driven for automated operation.
Manual filtering generally involves aspirating the sample into the syringe, removing the syringe needle, fitting a filter membrane to the front of the syringe, dispensing the fluid through the filter, removing the filter and fitting a needle to the syringe for the next sample. Often this process leads to spillage and occupational health and safety issues related to repetitive strain injury. The process is slow when performed manually but is also not an efficient process to automate.
Syringe filters typically have high dead volume resulting in the need for considerable prefiltered sample to obtain the required quantity of filtered sample.
While there is a strong need for increased focus on laboratory automation or simplification of sample preparation processes using conventional syringe filters these processes are not easily amendable to automation.
Solid Phase Extraction (SPE) is based on partitioning molecules between a solid stationary phase and liquid mobile phase (e.g. liquid chromatography). The technique of SPE is most commonly practised by loading the sample on the top of the solid stationary phase bed and the flow through the bed is either driven by gravity or vacuum assisted, which means the pressure differential across the bed is limited to atmospheric pressure. Pressure differential limitation means large particle size separation media (usually 35-50 micron) must be used. Smaller particle sizes are not practical using normal SPE techniques as the fluid flow is too restricted for both drawing fluids through the bed and dispensing the fluids. Smaller particle seizes for the media would offer significant advantages in terms of increased absorption capacity of the media (From larger surface area per volume), improved extraction efficiencies, and the possibility for more selective separation of compounds from either other on the bed in the same way that smaller particle size gives greater compound separation efficiency on a liquid chromatography column.
Conventional SPE cartridges require a relatively large amount of solvent to elute the compounds from the SPE bed. Typically evaporation of this excess extraction solvent is then required as an additional part of the process.
The cartridges also do not lend the process to simple automation due to method process, connectability and manual manipulation issues during operation.
Elevated pressure driven SPE has been practised by using gas pressure above the SPE bed.
A further development of SPE for sample preparation was Micro Extraction by Packed Sorbent (MEPS) (described for example in US patent publication US2004/0241874) which is a syringe-based design. The stationary phase bed is packed into the barrel of a syringe. The syringe is then used to draw a defined volume of sample through the bed with bed and solvent conditions chosen to trap targeted compounds on the bed. Targeted compounds are then eluted off the bed by aspirating a suitable solvent through the bed. The targeted compounds are thereby desorbed into the solvent and thereby carried into the barrel of the syringe. The solution containing the targeted compounds is then dispensed from the barrel, back through the bed and into a vial for analysis, or directly into an analytical instrument.
The MEPS technique had large advantages over conventional SPE including ease of automation and reduced volumes of elution solvent containing the targeted compounds, which gives greater concentration of the eluted compounds for analysis. The disadvantage remains of having to use large particle size separation media because the sample must be aspirated through the media using suction from the syringe. Also, when the elution solvent is drawn through the MEPS bed, the sample compounds are spread evenly through the elution volume from the syringe barrel, meaning the concentration factor of the sample in the eluted solvent is not as high as could be achieved.
The sample needs to be drawn through the bed where targeted compounds are trapped and non-trapped material is dispensed to waste back through the bed. The compounds are released from the bed when an elution solvent is aspirated through the sorbent bed bringing the elution solvent containing the targeted compounds into the barrel of the syringe. This solution is then dispensed through the bed out through the needle. It is significant that the compounds of interest are spread evenly through the dispensed volume so they are somewhat diluted over the entire elution volume rather than primarily in a concentrated band, which would give greater detection sensitivity.
The discussion thus far has focused on sample preparation techniques that employ syringes, but much of the discussion applies equally to the use of pipette tips, including disposable pipette tips.
It is an object of the invention to at least in part address or alleviate one or more of the difficulties mentioned above.
Reference to any prior art or background information in this specification is not, and should not be taken as, an acknowledgment or any form of suggestion that this prior art or background information forms part of the common general knowledge in Australia or any other jurisdiction; or that this prior art or background information could reasonably be expected to be ascertained, understood and regarded as relevant by a person skilled in the art.